Lean burn engines, such as diesel engines, produce an exhaust emission that generally contains at least four classes of pollutant that are legislated against by inter-governmental organisations throughout the world: carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM).
A variety of emissions control devices exist for the treatment of oxides of nitrogen (NOx). These devices include, for example, a selective catalytic reduction (SCR) catalyst, a selective catalytic reduction filter (SCRF™) catalyst, a lean NOx catalyst [e.g. hydrocarbon (HC) SCR catalyst], a lean NOx trap (LNT) [also known as a NOx storage catalyst (NSC) or a NOx adsorber catalyst (NAC)] and a passive NOx adsorber (PNA).
SCR catalysts or SCRF™ catalysts typically achieve high efficiencies for treating NOx by reduction once they have reached their effective operating temperature. However, these catalysts or devices can be relatively inefficient below their effective operating temperature, such as when the engine has been started from cold (the “cold start” period) or has been idling for a prolonged period.
Another common type of emissions control device for reducing or preventing the emission of NOx is a lean NOx trap (LNT). During normal operation, a lean burn engine produces an exhaust emission having a “lean” composition. An LNT is able to store or trap the nitrogen oxides (NOx) that are present in the “lean” exhaust emission. The LNT stores or traps the NOx present in the exhaust emission by a chemical reaction between the NOx and a NOx storage component of the LNT to form an inorganic nitrate. The amount of NOx that can be stored by the LNT is limited by the amount of NOx storage component that is present. Eventually, it will be necessary to release the stored NOx from the NOx storage component of the LNT, ideally when a downstream SCR or SCRF™ catalyst has reached its effective operating temperature. Release of stored NOx from an LNT is typically achieved by running the lean burn engine under rich conditions to produce an exhaust emission having a “rich” composition. Under these conditions, the inorganic nitrates of the NOx storage component decompose to reform NOx. This requirement to purge an LNT under rich conditions is a disadvantage of this type of emissions control device because it affects the fuel economy of the vehicle and it increases the amount of carbon dioxide (CO2) by combustion of additional fuel. LNTs also tend to show poor NOx storage efficiency at low temperatures.
A relatively new type of emissions control device for NOx is a passive NOx adsorber (PNA). PNAs are able to store or adsorb NOx at relatively low exhaust gas temperatures (e.g. less than 200° C.), usually by adsorption, and release NOx at higher temperatures. The NOx storage and release mechanism of PNAs is thermally controlled, unlike that of LNTs which require a rich purge to release stored NOx.